Microelectromechanical systems (MEMS) technology has achieved wide popularity in recent years, as it provides a way to make very small mechanical structures and integrate these structures with electrical devices on a single substrate using conventional batch semiconductor processing techniques. One common application of MEMS is the design and manufacture of sensor devices. MEMS sensors are widely used in applications such as automotive, inertial guidance systems, household appliances, game devices, protection systems for a variety of devices, and many other industrial, scientific, and engineering systems.
One particular type of MEMS sensor that is used in a variety of applications is an accelerometer. Typically, a MEMS accelerometer includes, among other component parts, a movable element, also referred to as a proof mass. The proof mass is resiliently suspended by one or more compliant suspension springs such that it moves when the MEMS accelerometer experiences acceleration. The motion of the proof mass may then be converted into an electrical signal having a parameter magnitude (e.g., voltage, current, frequency, etc.) that is proportional to the acceleration.
In some instances, a MEMS accelerometer may experience harsh accelerations or a relatively high force. In such an instance, the proof mass can move beyond a desired distance. Such movement can potentially damage the MEMS accelerometer and/or cause unstable behavior of the MEMS accelerometer. Accordingly, over-travel stops are typically used in accelerometers for limiting the excessive motion of the proof mass under relatively high acceleration. Various over-travel stops limit motion of the proof mass in three axes, i.e., two in-plane axes and one out-of-plane axis, in order to prevent circuit shortage, high wear rate, fracture, stiction, and so forth.
Referring to FIGS. 1-2, FIG. 1 shows a top view of a prior art inertial sensor, for example, an accelerometer 20 having over-travel stops 22, and FIG. 2 shows a sectional side view of accelerometer 20 along section lines 2-2 of FIG. 1. Accelerometer 20 includes a proof mass 24 suspended above and anchored to a substrate 26 via one or more proof mass anchors 28. More particularly, one or more compliant members 30, or springs, interconnect proof mass 24 with proof mass anchors 28. Fixed electrodes 32 (represented by dashed line boxes) underlie proof mass 24. Fixed electrodes 32, which may be some combination of sense electrodes and actuator electrodes, are formed or otherwise attached to substrate 26. Accelerometer 20 represents a typical single axis accelerometer. Accordingly, compliant members 30 enable movement of proof mass 24 about a rotational axis 34 when accelerometer 20 experiences acceleration in a z-direction 36 substantially perpendicular to the surface of substrate 26. Movement of proof mass 24 alters capacitances between proof mass 24 and fixed electrodes 32, and these capacitances are used to determine acceleration.
Over-travel stops 22 limit the movement, or deflection, of proof mass 24 when accelerometer 20 experiences harsh or excessive acceleration. In this example, over-travel stops 22 are configured to limit motion of proof mass 24 in three axes, two in-plane axes and one out-of-plane axis. Accordingly, each of over-travel stops 22 can include a lateral stop structure 38 and a cap 40. Lateral stop structure 38 is coupled to substrate 26 and limits in-plane deflection of proof mass 24. Cap 40 is coupled to lateral stop structure 38 and limits out-of-plane motion of proof mass 24.
Gaps 42 between lateral stop structures 38 and proof mass 24 are typically as small as one micron to limit the motion of proof mass 24 and prevent over stressing of proof mass 24. These gaps 42 are covered by caps 40. Fabrication operations can result in byproducts or debris falling into and getting lodged in gaps 42. This debris is referred to herein as particles 44. The small size of gaps 42, covered with caps 40, results in a situation in which particles 44 cannot be completely rinsed out of gaps 42 following fabrication processes. The residual particles 44 can prevent proof mass 24 from moving as a function of the acceleration, resulting in failure and yield loss for accelerometers.